Method of producing steel material, apparatus that cools steel material, and steel material

ABSTRACT

A method of producing a steel material, wherein when a cooling apparatus having a plurality of cooling sections disposed side by side in a longitudinal direction of a steel material cools the steel material hot worked or cooled/reheated, the steel material is conveyed at a conveyance distance L o  (m) satisfying Equation (1), in one direction along with the longitudinal direction of the steel material, in the cooling apparatus, wherein L o  is defined as conveyance distance (m) of steel material, m is a natural number, and L h  is defined as length (m) of cooling sections in longitudinal direction of steel material: 
       ( m− 0.20)× L   h   ≤L   o ≤( m +0.20)× L   h   (1).

TECHNICAL FIELD

This disclosure relates to a method of producing a steel material, anapparatus that cools steel material, and a steel material.

BACKGROUND

One of the longest steel materials is a rail for railways. Inparticular, a rail where a rail head section has a pearlite structurehigh in hardness is, for example, produced as follows.

First, bloom cast by continuous casting is reheated to 1100° C. or moreand, thereafter, hot rolled by rough rolling and finish rolling to havea predetermined rail shape. The rolling method in each rolling step isperformed by a combination of caliber rolling and universal rolling, orby only caliber rolling, and such rough rolling is performed for aplurality of passes and such finish rolling is performed for a pluralityof passes or a single pass. The rail here usually has a length of about50 m to 200 m by hot rolling.

Next, an unsteady section at an end of the rail hot rolled is hot sawn(hot sawing step). When a heat treatment apparatus is here limited withrespect to the length, further sawing is performed so that apredetermined length (for example, 25 m) is achieved.

After the hot sawing step, a coolant (air, water, mist, or the like) issprayed to the rail in a cooling apparatus, thereby performing forcedcooling (heat treatment step). In the heat treatment step, the rail isrestricted by a restraint apparatus such as a clamp, and the coolant issprayed to a head section, a foot section, and also, if necessary, aweb. The cooling apparatus usually performs cooling until thetemperature of the head section of the rail reaches 650° C. or less.After such forced cooling is completed, the rail is released from therestraint apparatus, and further conveyed to a cooling bed and cooled to100° C. or less.

When the rail for railways is, for example, a rail for use in a severeenvironment where heavy goods such as coal and iron ore are transportedfrom mines having natural resources such as coal, such a rail isdemanded to have high wear resistance and high toughness and, therefore,the heat treatment step is required. The heat treatment is performed,thereby enabling the rail to be high in hardness and decreasing theamount of wear in use and, therefore, the effects of increasing the railreplacement period and decreasing the life-time cost are achieved. Whenthe variation in hardness is large in the longitudinal direction of therail, however, is not preferable because the amount of wear is larger ata low-hardness section than a high-hardness section, thereby not onlyincreasing the vibration in train running, but also decreasing thereplacement period. Thus, there is demanded a heat treatment method thatallows the rail to be small in the variation in hardness and high inhardness.

For example, JP H03-166318 A discloses a method of suppressing a coolingrate to 7° C./sec or less, as a method of decreasing the variation inhardness of a rail.

Moreover, J P 2003-193126 A discloses a method of oscillating anH-shaped steel in an amount obtained by an Equation with the pitchbetween nozzles being adopted as a parameter in accelerated cooling ofthe H-shaped steel, as a method of uniformly cooling a steel material.Furthermore, J P 2006-55864 A discloses a method of oscillating a steelmaterial at a distance 5 times to 10 times the distance in thelongitudinal direction of the material of a guide roller, as a method ofuniformly cooling a steel material.

The method described in JP '318 can decrease the influence of thevariation in temperature at the start of a heat treatment in thelongitudinal direction of a steel material on the variation in hardness.In the heat treatment, however, when the variation in cooling rate iscaused in the longitudinal direction of a steel material, uniformhardness is not achieved. Therefore, it is difficult to produce a steelmaterial uniform in material properties in the longitudinal direction.

While the methods described in JP '126 and JP '864 can alleviate thereduction in cooling rate due to a weak cooling section generated incooling equipment, it is difficult to provide a uniform cooling ratewhen the variation in cooling rate is caused between cooling headers inthe longitudinal direction of a steel material. Therefore, it isdifficult to produce a steel material uniform in material propertiessuch as hardness in the longitudinal direction.

It could therefore be helpful to provide a method of producing a steelmaterial uniform in material properties in the longitudinal direction,an apparatus that cools steel material, and a steel material.

SUMMARY

We thus provide:

A method of producing a steel material, wherein, when a coolingapparatus having a plurality of cooling sections disposed side by sidein the longitudinal direction of a steel material cools the steelmaterial hot worked or cooled/reheated, the steel material is conveyedat a conveyance distance L_(o) (m) satisfying Equation (1), in onedirection along with the longitudinal direction of the steel material,in the cooling apparatus:

(m−0.20)×L _(h) ≤L _(o)≤(m+0.20)×L _(h)  (1)

L_(o): conveyance distance (m) of steel material.m: natural numberL_(h): length (m) of cooling sections in longitudinal direction of steelmaterial.

An apparatus that cools steel material hot worked or cooled/reheated,including: a plurality of cooling sections disposed side by side in thelongitudinal direction of the steel material; and a conveyance sectionthat conveys the steel material at a conveyance distance L_(o) (m)satisfying Equation (1), in one direction along with the longitudinaldirection of the steel material in the cooling apparatus, during coolingof the steel material in the cooling sections.

A steel material produced by hot working or cooling/reheating andthereafter cooling in a cooling apparatus having a plurality of coolingsections disposed side by side in a longitudinal direction, wherein,during cooling in the cooling apparatus, the steel material is producedwith being conveyed at a conveyance distance L_(o) (m) satisfyingEquation (1), in one direction along with the longitudinal direction ofthe steel material in the cooling apparatus.

We can provide a method of producing a steel material uniform inmaterial properties in the longitudinal direction, an apparatus thatcools steel material, and a steel material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating a cooling apparatus according toone example.

FIG. 2 is a cross-section view illustrating each section of a rail.

FIG. 3 is a plan view illustrating peripheral equipment of the coolingapparatus.

FIG. 4A and FIG. 4B are schematic views illustrating a conveyanceoperation of the cooling apparatus.

FIG. 5 is a plan view illustrating peripheral equipment of a coolingapparatus in Examples.

FIG. 6 is a schematic view illustrating a conveyance state on adischarge table in Examples.

REFERENCE SIGNS LIST

-   1 rail-   11 head section-   12 web section-   13 foot section-   2 cooling apparatus-   21 a to 21 c head section-cooling header-   22 foot section-cooling header-   23 a, 23 b clamp-   24 thermometer in apparatus-   25 conveyance section-   3 carrying-in table-   4 discharge table-   5 exit side thermometer

DETAILED DESCRIPTION

In the following detailed description, many particular details aredescribed to provide a complete understanding of examples of ourmethods, cooling apparatus and steel materials. It, however, will beapparent that one or more aspects can be carried out even without suchparticular details. Additionally, well-known configurations andapparatuses are schematically illustrated to simplify the drawings.

Configuration of Cooling Apparatus

First, an apparatus 2 that cools steel material according to one exampleis described with reference to FIGS. 1 to 3. Herein, a rail 1 isproduced as a steel material. The cooling apparatus 2 is used in a heattreatment step performed after a hot rolling step or a hot sawing stepdescribed below, and forcedly cools a rail 1 having a high temperature.The rail 1, when viewed cross-sectionally perpendicular to thelongitudinal direction, includes a head section 11 and a foot section 13extending in the width direction and opposite to each other in thevertical direction, and a web section 12 connecting the head section 11disposed above and the foot section 13 disposed below and extending inthe vertical direction, as illustrated in FIG. 2.

As illustrated in FIG. 1, the cooling apparatus 2 includes headsection-cooling headers 21 a to 21 c, a foot section-cooling header 22,a pair of clamps 23 a and 23 b, a thermometer 24 in the apparatus, and aconveyance section 25. The head section-cooling headers 21 a to 21 c,and the foot section-cooling header 22 serve as cooling sections to coolthe rail 1, and a plurality of the respective headers are providedcontinuously side by side in the y-axis direction serving as thelongitudinal direction of the rail 1. In the following description, thehead section-cooling headers 21 a to 21 c, and the foot section-coolingheader 22 are also collectively called cooling headers.

The head section-cooling headers 21 a to 21 c have coolant-sprayingoutlets arranged at pitches of several mm to 100 mm, and thecoolant-spraying outlets of each of the head section-cooling headers 21a to 21 c are provided oppositely on each of the head top surface (endsurface in the z-axis positive direction) and the head side surfaces(both end surfaces in the x-axis positive direction) of the head section11. The head section-cooling headers 21 a to 21 c each spray a coolantsupplied from a supply section not illustrated, to the head top surfaceand the head side surface of the head section 11, thereby subjecting thehead section 11 to forced cooling. The coolant to be used is air, spraywater, mist or the like. Respective pressure measurement apparatuses 211a to 211 c are also provided on coolant supply pathways of the headsection-cooling headers 21 a to 21 c, and the coolant spray pressure ismonitored.

The foot section-cooling header 22 has coolant-spraying outlets arrangedat pitches of several mm to 100 mm, and the coolant-spraying outlets areprovided opposite to the lower surface (end surface in the z-axisnegative direction) of the foot section 13. The foot section-coolingheader 22 sprays a coolant supplied from a supply section notillustrated, to the lower surface of the foot section 13, therebysubjecting the foot section 13 to forced cooling, as in the headsection-cooling headers 21 a to 21 c. The coolant to be used is air,spray water, mist or the like, as in the head section-cooling headers 21a to 21 c. A pressure apparatus 221 is also provided on a coolant supplypathway of the foot section-cooling header 22, and the coolant spraypressure is monitored.

The head section-cooling headers 21 a to 21 c and the footsection-cooling header 22 each have the same length in the y-axisdirection. The cooling headers are heated from the rail 1 and thusthermally deformed, thereby causing warpage (the generation mechanism ofsuch warpage is described below) to be generated. The amount of warpageof the cooling headers, generated at the same curvature, increases withthe square of the length of the cooling headers in the z-axis direction.Therefore, the length of the cooling headers in the z-axis direction ispreferably shorter. On the other hand, an increase in the number of thecooling headers provided in the z-axis direction for a decrease in thelength of the cooling headers is not preferable because there arerequired many feed ports of the coolant as well as many measurementdevices and control devices of the amount of coolant spray (for example,a pressure gauge, a flow meter, and a flow regulator) mounted to thecooling headers and a pipe arrangement. Accordingly, the length of thecooling headers in the z-axis direction is needed to be a proper length,and is preferably 0.5 m or more and 4 m or less. The headsection-cooling headers 21 a to 21 c and the foot section-cooling header22 provided side by side in the y-axis direction are preferably providedas close as possible so that any cooling irregularity is not caused.

The pair of clamps 23 a and 23 b is an instrument that sandwiches eachof both ends of the foot section 13 in the x-axis direction to therebysupport and restrain the rail 1. The pair of clamps 23 a and 23 b isplurally provided over the entire length in the longitudinal directionof the rail 1 and are several meters apart.

The thermometer 24 in the apparatus is a non-contact thermometer such asa radiation thermometer, and measures the surface temperature of atleast one point on the head top surface of the head section 11.

The conveyance section 25 is a conveyance mechanism connected to thepair of clamps 23 a and 23 b, and is an apparatus that conveys the pairof clamps 23 a and 23 b in the y-axis direction, thereby conveying therail 1 in the cooling apparatus 2. The detail of a conveyance operationof the conveyance section 25 is described below.

In the cooling apparatus 2 configured above, the amount of the coolantsprayed from each of the head section-cooling headers 21 a to 21 c andthe foot section-cooling header 22 is adjusted by a control section notillustrated. The control section here acquires the temperaturemeasurement result of the thermometer 24 in the apparatus, and theamount sprayed is adjusted, as needed, based on the temperaturemeasurement result acquired.

As illustrated in FIG. 3, a carrying-in table 3 and a discharge table 4are provided on the periphery of the cooling apparatus 2. Thecarrying-in table 3 is a table that conveys the rail 1 from a precedingstep such as the hot rolling step to the cooling apparatus 2. Thedischarge table 4 is a table that conveys the rail 1 heat-treated in thecooling apparatus 2, to a next step such as a cooling bed or anexamination instrument. An exit side thermometer 5 is a non-contactthermometer that measures the surface temperature of the head section 11of the rail 1, as in the thermometer 24 in the apparatus, and thatmeasures the temperature of the rail 1 discharged from the coolingapparatus 2 after the heat treatment.

Method of Producing Steel Material

Next, a method of producing a steel material according to an example isdescribed. A perlite-based rail 1 is produced as a steel material. Therail 1 that can be used is, for example, steel including the followingchemical component composition. Herein, Equation by “%” with respect toeach chemical component means “% by mass,” unless especially noted.

C: 0.60% or more and 1.05% or less

C (carbon) is an important element that forms cementite in aperlite-based rail, resulting in increases in hardness and strength andenhancement in wear resistance. If the C content is less than 0.60%,however, such effects are less exerted. The C content is thus preferably0.60% or more, more preferably 0.70% or more. On the other hand, if C isexcessively contained, an increase in the amount of the cementite can beachieved to result in increases in hardness and strength, butdeterioration in ductility is conversely caused. Moreover, an increasein the C content expands the temperature range of the y+0 region, andpromotes softening of a welded heat affected zone. In consideration ofsuch adverse effects, the C content is preferably 1.05% or less, morepreferably 0.97% or less.

Si: 0.1% or more and 1.5% or less

Si (silicon) is added to enhance a deoxidizer and a pearlite structurein a rail material, but such an effect is less exerted if the content isless than 0.1%. Therefore, the Si content is preferably 0.1% or more,more preferably 0.2% or more. On the other hand, if Si is excessivelycontained, decarburization is promoted and generation of surface defectsof the rail 1 is promoted. Therefore, the Si content is preferably 1.5%or less, more preferably 1.3% or less. Mn: 0.01% or more and 1.5% orless

Mn (manganese) has the effects of decreasing the temperature of perlitetransformation and finning the perlite lamellar spacing and, therefore,is an element effective to maintain high hardness inside the rail 1. Ifthe Mn content is less than 0.01%, however, the effects are lessexerted. Therefore, the Mn content is preferably 0.01% or more, morepreferably 0.3% or more. If the Mn content is more than 1.5%, theequilibrium transformation temperature (TE) of perlite is lowered, andmartensitic transformation easily occurs in the structure. Therefore,the Mn content is preferably 1.5% or less, more preferably 1.3% or less.P: 0.035% or less

P (phosphorus) causes deterioration in toughness and ductility, if thecontent thereof is more than 0.035%. Therefore, the P content ispreferably made lower. Specifically, the P content is preferably 0.035%or less, more preferably 0.025% or less. If special refining or the likeis here performed to decrease the P content as much as possible, costrise is caused in smelting. Therefore, the P content is preferably0.001% or more.

S: 0.030% or less

S (sulfur) forms coarse MnS extending in the rolling direction andresulting in deterioration in ductility and toughness. Therefore, the Scontent is preferably made lower. Specifically, the S content ispreferably 0.030% or less, more preferably 0.015% or less. If the Scontent is here decreased as much as possible, cost rise in smelting isremarkably caused due to increases in smelting treatment time and theamount of a solvent. Therefore, the S content is preferably 0.0005% ormore.

Cr: 0.1% or more and 2.0% or less

Cr (chromium) increases the equilibrium transformation temperature (TE),contributes to fining of the perlite lamellar spacing, and increaseshardness and strength. Cr, when used in combination with Sb, is alsoeffective in inhibiting a decarburization layer from being generated.Therefore, the Cr content is preferably 0.1% or more, more preferably0.2% or more. If the Cr content is more than 2.0%, not only thepossibility of the occurrence of welding defects is increased, but alsohardenability is increased, and generation of martensite is promoted.Therefore, the Cr content is preferably 2.0% or less, more preferably1.5% or less.

The total content of Si and Cr is desirably 2.0% or less. The reason isbecause, if the total content of Si and Cr is more than 2.0%, anexcessive increase in scale adhesiveness can inhibit scale peeling andpromote decarburization.

Sb: 0.005% or more and 0.5% or less

Sb (antimony) has a remarkable effect of preventing decarburizationduring heating of a rail steel material in a heating furnace. Inparticular, Sb is added together with Cr, to thereby have the effect ofreducing generation of a decarburization layer, when the Sb content is0.005% or more. Therefore, the Sb content is preferably 0.005% or more,more preferably 0.01% or more. If the Sb content is more than 0.5%, theeffect is saturated. Therefore, the Sb content is preferably 0.5% orless, more preferably 0.3% or less.

The steel for use as the rail 1 may further contain, in addition to thechemical composition, one or more elements of Cu: 0.01% or more and 1.0%or less, Ni: 0.01% or more and 0.5% or less, Mo: 0.01% or more and 0.5%or less, V: 0.001% or more and 0.15% or less, and Nb: 0.001% or more and0.030% or less.

Cu: 0.01% or more and 1.0% or less

Cu (copper) is an element that can provide much higher hardness by solidsolution strengthening. Cu also has the effect of suppressingdecarburization. To expect such an effect, the Cu content is preferably0.01% or more, more preferably 0.05% or more. If the Cu content is morethan 1.0%, surface cracking due to embrittlement in continuous castingand/or rolling easily occurs. Therefore, the Cu content is preferably1.0% or less, more preferably 0.6% or less.

Ni: 0.01% or more and 0.5% or less

Ni (nickel) is an element effective to enhance toughness and ductility.Moreover, Ni is an element also effective to suppress Cu cracking byaddition as a composite with Cu. Therefore, when Cu is added, Ni isdesirably added, and the Ni content is more preferably 0.05% or more. Ifthe Ni content is less than 0.01%, however, such effects are notexerted. Therefore, the Ni content is preferably 0.01% or more. If theNi content is more than 0.5%, hardenability is increased, and generationof martensite is promoted. Therefore, the Ni content is preferably 0.5%or less, more preferably 0.3% or less.

Mo: 0.01% or more and 0.5% or less

Mo (molybdenum) is an element effective for an increase in strength, butsuch an effect is less exerted if the content is less than 0.01%.Therefore, the Mo content is preferably 0.01% or more, more preferably0.05% or more. If the Mo content is more than 0.5%, an increase inhardenability causes martensite to be generated, resulting in extremedeterioration in toughness and ductility. Therefore, the Mo content ispreferably 0.5% or less, more preferably 0.3% or less.

V: 0.001% or more and 0.15% or less

V (vanadium) is an element that forms VC, VN or the like and is finelyprecipitated in ferrite, and that contributes to an increase in strengththrough precipitation strengthening. V can also be expected to have theeffects of serving as a trap site of hydrogen and suppressing delayedfracture. To exert such effects, the V content is preferably 0.001% ormore, more preferably 0.005% or more. If V is added in a rate of morethan 0.15%, an increase in alloy cost is remarkable relative tosaturation of such effects. Therefore, the V content is preferably 0.15%or less, more preferably 0.12% or less.

Nb: 0.001% or more and 0.030% or less

Nb (niobium) is effective to allow the unrecrystallized temperatureregion of austenite to be in a higher temperature region and promotingintroduction of processing strain into austenite in rolling, therebyfining the sizes of perlite colony and block. Thus, Nb is an elementeffective for enhancements in ductility and toughness. To exert sucheffects, the Nb content is preferably 0.001% or more, more preferably0.003% or more. If the Nb content is more than 0.030%, Nb carbonitrideis crystalized in the course of solidification in casting of a railsteel material such as bloom, resulting in deterioration in cleanliness.Therefore, the Nb content is preferably 0.030% or less, more preferably0.025% or less.

The balance other than the above components is configured from Fe (iron)and inevitable impurities. Up to 0.015% of N (nitrogen), up to 0.004% ofO (oxygen), and up to 0.0003% of H (hydrogen) can be allowed to beincorporated as such inevitable impurities. To suppress deterioration inrolling fatigue characteristics due to hard AlN and TiN, the Al contentis desirably 0.001% or less and the Ti content is desirably 0.001% orless.

In a method of producing the rail 1 according to the example, first, forexample, the bloom of the chemical component composition, serving as thematerial of the rail 1 cast by a continuous casting method, is carriedin a heating furnace, and heated to 1100° C. or more.

Next, the bloom heated is rolled in each of a break-down roller, a roughroller and a finish roller for one or more passes, and finally rolled tothe rail 1 having a shape illustrated in FIG. 2 (hot rolling step). Thelength in the longitudinal direction of the rail 1 rolled is here about50 m to 200 m, and is, if necessary, hot sawn to have a length of, forexample, 25 m (hot sawing step). A shorter length in the longitudinaldirection of the rail 1 here causes the subsequent heat treatment stepto be involuntarily affected by the coolant sprayed onto the end surfacein the longitudinal direction during cooling. Therefore, the length inthe longitudinal direction of the rail 1 for use in the heat treatmentstep is three times or more the height from the head top surface of thehead section 11 of the rail 1 to the lower surface of the foot section13 thereof. On the other hand, the upper limit of the length in thelongitudinal direction of the rail 1 for use in the heat treatment stepis defined as the length of rolling (the maximum rolling length in thehot rolling step).

The hot rolled or hot sawn rail 1 is conveyed to the cooling apparatus 2by the carrying-in table 3, and cooled in the cooling apparatus 2 (heattreatment step).

The temperature of the rail 1 here conveyed to the cooling apparatus 2is desirably in the austenite temperature region. A rail for use in amine or a curved section is needed to have high hardness and, therefore,rapid acceleration is needed in the cooling apparatus 2 after rolling.Such acceleration fines the perlite lamellar spacing, thereby providinga high-hardness structure, and an increase in the degree of supercoolingin transformation, namely, an increase in the cooling rate intransformation can provide such a high-hardness structure. If thestructure of the rail 1, however, is transformed before cooling in thecooling apparatus 2, such transformation progresses at an extremely lowcooling rate in spontaneous cooling and, therefore, cannot provide ahigh-hardness structure. Accordingly, when the temperature of the rail 1is equal to or lower than the lowest temperature in the austenitetemperature region at the start of cooling in the cooling apparatus 2,the rail 1 is preferably reheated to any temperature in the austenitetemperature region and thereafter subjected to the heat treatment step.

In the heat treatment step, the rail 1 is conveyed to the coolingapparatus 2, and thereafter the rail 1 is restrained by the clamps 23 aand 23 b. Thereafter, the rail 1 is rapidly cooled by spraying thecoolant from each of the head section-cooling headers 21 a to 21 c andthe foot section-cooling header 22. The cooling rate in the heattreatment is preferably changed depending on the desired hardness and,furthermore, the cooling rate may be excessively increased, therebycausing martensitic transformation to occur and impairing toughness.Therefore, the control section monitors the cooling rate based on theresult of the temperature measured by the thermometer 24 in theapparatus during cooling, and changes the amount of the coolant to besprayed. The control section may also be here, if necessary, set to stopspraying of the coolant and perform cooling by spontaneous cooling.

In the heat treatment step, when a plurality of the cooling headersserving as the cooling sections of the cooling apparatus 2 have beenprovided in portions in the longitudinal direction of the rail,temperature variation has occurred in the longitudinal direction of therail 1 in some cases. We investigated the cause of the occurrence of thetemperature variation, and found as follows. The cooling headers may beclose to the rail 1 to achieve a high cooling rate in cooling of therail 1 having a high temperature. In such a case, the cooling headersare heated by radiation from the rail 1 and/or heat conduction of air,and therefore thermally deformed. Only surfaces of the cooling headers,the surfaces being closer to the steel material, are heated andthermally expended and, therefore, the cooling headers are usuallywarped so that end portions thereof are away from the rail 1. When thecooling headers are thus deformed, the end portions are away from therail 1 against the center portion of the cooling headers, therebyresulting in a reduction in the cooling rate at the end portions ascompared with the center portion. Therefore, a strong cooling sectionand a weak cooling section are repeatedly present in the longitudinaldirection of the rail 1 at an interval where each of the cooling headersis provided, thereby causing the temperature variation in thelongitudinal direction of the rail 1.

We found that such temperature variation can be eliminated byoscillating the rail 1 in the cooling apparatus 2 along with thelongitudinal direction of the rail 1 at a predetermined amplitude andconveying it. In other words, in the heat treatment step, the conveyancesection 25 conveys the clamps 23 a and 23 b together with the rail 1restrained, with oscillation at a predetermined amplitude, in cooling.Such oscillation here means an operation that conveys the rail 1alternately in the y-axis positive direction and in the y-axis negativedirection by a predetermined conveyance distance L_(o). The conveyancedistance L_(o) serving as the amplitude of oscillation corresponds tothe distance (m) satisfying Equation (1). In Equation (1), m representsa natural number, and L_(h) represents the length (m) of the coolingheaders, being the length of the cooling sections in the longitudinaldirection of the rail 1 (y-axis direction), respectively:

(m−0.20)×L _(h) ≤L _(o)≤(m+0.20)×L _(h)  (1).

The conveyance operation of the rail 1 by the conveyance section 25 isdescribed with reference to FIG. 4. In the example illustrated in FIG.4, the conveyance distance L_(o) in the heat treatment step is a lengthtwice the length L_(h) of the cooling headers (head section-coolingheader 21 a and foot section-cooling header 22) serving as the coolingsections. The conveyance section 25 then conveys the rail 1 in the stateillustrated in FIG. 4A at the conveyance distance L_(o) in the y-axisnegative direction. Thus, the rail 1 is in the state illustrated in FIG.4B from the state illustrated in FIG. 4A. Next, the conveyance section25 conveys the rail 1 in the state illustrated in FIG. 4B at theconveyance distance L_(o) in the y-axis positive direction. Thus, therail 1 is again in the state illustrated in FIG. 4A from the stateillustrated in FIG. 4B. Such operations are repeated to perform theconveyance operation.

Furthermore, the conveyance operation of the rail 1 in the coolingapparatus 2 by the conveyance section 25 is preferably performedcontinuously during cooling of the rail 1. In other words, when thecooling time of the rail 1 in the heat treatment step is defined as T(min), the conveyance velocity V (mm/min) of the rail 1 is set so thatEquation (2) is satisfied. In Equation (2), n represents a naturalnumber:

V=L _(h)/(T×n)  (2).

Furthermore, cooling is performed in the heat treatment step until afinal structure made of 100% of perlite, or a final structure having 5%or less of pro-eutectoid ferrite and pro-eutectoid cementite and thebalance being perlite or a final structure where perlite and bainite aremixed is obtained. The bainite phase and the cementite phase areimpaired in toughness, therefore, a structure made of 100% of theperlite phase is preferable to not generate any failures caused bydeterioration in toughness such as sharing, and a final structure isdetermined depending on the intended use.

As described above, a high-hardness structure is obtained by allowingtransformation to occur in the heat treatment and, therefore, the heattreatment completion temperature is needed to be achieved aftercompletion of transformation. While the depth necessary for such ahigh-hardness structure, however, varies depending on the intended useof the rail 1 and the heat treatment completion temperature cannot bethus clearly limited, cooling is needed to be performed at least untilthe temperature of the surface of the head section 11 reaches 650° C. orless.

After the heat treatment step, the rail 1 is conveyed to the cooling bedby the discharge table 4, and cooled thereon to a temperature rangingfrom room temperature to 100° C. Thereafter, the rail 1 is straightenedby roller straightening to decrease warpage. The rail 1 then undergoesan examination and thereafter is shipped. Since a sectionnon-straightened is generated at an end in the longitudinal direction ofthe rail 1 in straightening by roller straightening, cold sawing mayalso be performed after straightening by roller straightening, withoutsawing to the length of a final product in hot sawing. The end in thelongitudinal direction of the rail 1, in cold sawing, here correspondsto each of both ends in the rolling length and, therefore, any sectionnot-straightened is decreased and warpage is decreased.

A rail 1 uniform in material properties in the longitudinal directioncan be produced through the above steps.

Modifications

Although our methods, cooling apparatus and materials are describedabove with reference to particular examples, this disclosure is notintended to be limited by such description. Not only variousmodifications of the examples disclosed, but also other examples arealso apparent to those skilled in the art with reference to the detaileddescription. Accordingly, it is to be understood that the appendedclaims also cover such modifications or examples encompassed in thescope and gist of this disclosure.

For example, the rail 1 is used as the steel material in the example,but the disclosure is not limited to such an example. For example, thesteel material to be produced may be any other steel material productsuch as a thick plate or a shaped steel. In such a case, the chemicalcomponent composition of the steel material product, the configurationof the cooling apparatus 2 and the like are not limited to the examples.Even when the steel material to be produced is the rail 1, any steelhaving a different chemical component composition from that in theexample may be used. As described above, an end surface is involuntarilyaffected by the coolant sprayed, during cooling and, therefore, theminimum length in the longitudinal direction of the steel materialproduct is three times or more the thickness of the thickest portion ofa steel material such as a shaped steel, or three times or more thethickness of a plate material representative of a thick plate, and themaximum length thereof is the rolling length.

While the conveyance distance L_(o) satisfies Equation (1) in theexample, the conveyance distance L_(o) is preferably a value closer tothe integral multiple of the length L of the cooling sections, andpreferably satisfies Equation (3):

(m−0.10)×L _(h) ≤L _(o)≤(m+0.10)×L _(h)  (3).

Thus, the variation in cooling rate, caused in each header unit of thecooling sections, can be more decreased.

While the conveyance section 25 conveys the rail 1 with the rail 1 beingoscillated in the heat treatment step in the example, this disclosure isnot limited to such an example. For example, the conveyance section 25may be configured to convey the rail 1 at the conveyance distance L_(o)in only any one direction of the y-axis positive direction and they-axis negative direction with the rail 1 being not oscillated.

While the conveyance operation of the rail 1 in the cooling apparatus 2by the conveyance section 25 in the heat treatment step is continuouslyperformed during cooling of the rail 1 in the example, this disclosureis not limited to such an example. For example, the conveyance operationof the rail 1 in the example may be performed for a time more than halfof the cooling time T, after cooling of the rail 1. The conveyanceoperation is here performed at the conveyance distance L_(o) satisfyingEquation (1), for a predetermined time (time more than half of thecooling time T) from the start of cooling of the rail 1. Thereafter, theconveyance operation is preferably continuously performed for theremaining time of the cooling time T, but the conveyance distance L_(o)does not necessarily satisfy Equation (1). Thus, the time for whichuniform cooling can be made can be at least half of the heat treatmenttime, thereby decreasing the variation in cooling rate. In such a case,the conveyance velocity V does not necessarily satisfy Equation (2) and,therefore, application to a cooling apparatus 2 that cannot be changedin the conveyance velocity V can also be made.

Effects

(1) In a method of producing a steel material according to one example,when a cooling apparatus 2 having a plurality of cooling sections (headsection-cooling headers 21 a to 21 c, and a foot section-cooling header22) disposed side by side in the longitudinal direction of a steelmaterial cools a steel material hot worked or cooled/reheated, the steelmaterial is conveyed at the conveyance distance L_(o) (m) satisfyingEquation (1), in the longitudinal direction of the steel material, inthe cooling apparatus 2.

While the steel material is needed to be cooled at a high cooling rateto provide a high-hardness steel material, as described above, thecooling headers of the cooling apparatus 2 are needed therefor to becooled by being closer to the steel material. The cooling headers arehere heated by radiation or the like from the steel material, and thecooling headers are deformed to be warped in the longitudinal direction.If cooling is performed in such a state, the difference in distance fromthe steel material is caused in the longitudinal direction of thecooling headers, and thus the variation in the cooling rate (in a strongcooling section and a weak cooling section) is caused in each coolingheader unit, resulting in the occurrence of the variation in hardness ofthe steel material. For example, in production of the rail 1 as thesteel material, the rail 1 may be usually cooled with being oscillatedat a lower amplitude than that in the example, in the longitudinaldirection. The cooling rate is here higher at a position immediatelybelow each coolant-spraying outlet and lower at a position away from theposition immediately below each coolant-spraying outlet and, therefore,the rail can be at least conveyed at a distance (several mm to 100 mm)between coolant-spraying outlets, thereby uniformly passing through theposition immediately below each coolant-spraying outlet, higher in thecooling rate, and the position away therefrom, lower in the coolingrate. Such conventional oscillation (conveyance operation), however, hasnot be able to eliminate cooling irregularity caused in each coolingheader unit.

On the other hand, the above configuration can allow the steel materialto be conveyed at a distance substantially integral multiple of thelength L_(h) of the cooling headers in the longitudinal direction duringcooling, thereby allowing respective times, at which the steel materialpasses through the strong cooling section and the weak cooling section,to be the same at each position in a region corresponding to theconveyance distance L_(o) in the longitudinal direction of the steelmaterial. Therefore, the variation in cooling rate, caused in eachcooling header unit, can be decreased, thereby allowing a steel materialuniform in material properties such as hardness in the longitudinaldirection to be obtained. Furthermore, the distance between the coolingheaders and the steel material can be shorter and, therefore, a highcooling rate can be achieved and the steel material can have highhardness.

(2) In conveyance of the steel material in configuration (1) above, thesteel material is conveyed with being oscillated, and the amplitude ofsuch oscillation is set at the conveyance distance L_(o) satisfyingEquation (1).

Such a configuration can allow a long total conveyance distance to beachieved even when the length of the cooling apparatus does not havesufficient margin relative to the length in the longitudinal directionof the steel material.

(3) In Configuration (1) or (2), the Steel Material is a Rail Material.

Such a configuration can allow a rail material less in the variation inmaterial properties in the longitudinal direction to be obtained as arail material being a long steel material. For example, when the railmaterial is a high-hardness rail 1, the variation in cooling in the heattreatment step can be suppressed within 20° C. or less, and as a result,the variation in hardness can be suppressed within an HV of 13 or lessat a depth position of 1 mm from the surface and within an HV of 10 orless at a depth position of 5 mm therefrom.

(4) An apparatus 2 that cools steel material according to one example isa cooling apparatus 2 that cools steel material hot worked orcooled/reheated, including a plurality of cooling sections (headsection-cooling headers 21 a to 21 c, and a foot section-cooling header22) disposed side by side in the longitudinal direction of the steelmaterial, and a conveyance section 25 that conveys the steel material atthe conveyance distance L_(o) (m) satisfying Equation (1), in thelongitudinal direction of the steel material in the cooling apparatus 2,during cooling of the steel material in the cooling sections.

Such a configuration can allow the same effect as in configuration (1)above to be obtained.

(5) A steel material according to one example is a steel materialproduced by hot working or cooling/reheating and thereafter cooling in acooling apparatus 2 having a plurality of cooling sections (headsection-cooling headers 21 a to 21 c, and a foot section-cooling header22) disposed side by side in the longitudinal direction, wherein, duringcooling in the cooling apparatus 2, the steel material is produced withbeing conveyed at the conveyance distance L_(o) (m) satisfying Equation(1), in one direction along with the longitudinal direction of the steelmaterial in the cooling apparatus 2.

Such a configuration can allow the steel material to be uniformly cooledin the longitudinal direction, thereby providing a steel materialuniform in material properties in the longitudinal direction.

Example 1

Next, Example 1 is described. First, before Example 1, a rail 1 being asteel material was produced in a different conveyance distance L_(o)condition from the example, as Conventional Examples, and the materialproperties thereof were evaluated.

In the Conventional Examples, first, a bloom of a chemical componentcomposition in Condition A represented in Table 1 was cast by using acontinuous casting method. The balance of the chemical componentcomposition of the bloom was here substantially Fe, specifically Fe andinevitable impurities.

TABLE 1 Chemical component composition (% by mass) Condition C Si Mn P SAl Ti A 0.83 0.52 0.51 0.015 0.008 0.0005 0.001 B 0.83 0.52 1.11 0.0150.008 0.0005 0.001 C 1.03 0.52 1.11 0.015 0.008 0.0005 0.001

Next, the bloom cast was reheated to 1100° C. or more in a heatingfurnace, thereafter taken out from the heating furnace, and hot rolledthrough a break-down roller, a rough roller and a finish roller so thatthe cross-sectional shape was the final shape (rail shape illustrated inFIG. 2). In such hot rolling, the rail 1 was rolled at an invertedposition where a head section 11 and a foot section 13 were in contactwith a conveyance stage.

Furthermore, the rail 1 hot rolled was conveyed to a cooling apparatus2, and the rail 1 was cooled (heat treatment step). Since the rail 1 washere rolled at the inverted position as a rolling position, the rail 1,when carried in the cooling apparatus 2, was inverted, and was allowedto be at an erect position illustrated in FIG. 2, where the foot section13 was located below in the vertical direction and the head section 11was located above in the vertical direction, and the rail 1 wasrestrained by clamps 23 a and 23 b. Cooling was then performed byspraying of a coolant from each cooling header. During such cooling, thecoolant was air, and the distance between the cooling headers and therail was 20 mm or 50 mm. As disclosed in JP '318, the spray pressure ofthe coolant was set at 1.3 kPa to 130 kPa so that the cooling rate at670° C. to 770° C. at a depth position of 5 mm from the surface layerwas 3° C./sec to 7° C./sec, and cooling was performed until the surfacetemperature of the head section 11 reached 530° C. or less, whiletemperature measurement was performed by a thermometer 24 in theapparatus.

During cooling in the cooling apparatus 2, such cooling was performed ina condition where the rail 1 was not conveyed at all and in a conditionwhere the rail 1 was conveyed at a conveyance distance L_(o) of 1 m, inthe Conventional Examples. The length L_(h) of the cooling headers was 4m, and the rail 1 was conveyed at only a total distance of 4 m withbeing oscillated in the cooling apparatus 2, in the condition where therail 1 was conveyed.

After completion of the heat treatment, the rail 1 was taken out fromthe cooling apparatus 2 onto a discharge table 4, and the surfacetemperature of the head section 11 of the rail 1 after cooling wasmeasured by use of an exit side thermometer 5 provided on the dischargetable 4 as illustrated in FIGS. 5 and 6. The exit side thermometer 5 washere used to measure the temperature at a plurality of positions overthe entire length in the longitudinal direction of the rail 1, and thevariation in temperature after cooling was calculated from the maximumvalue and the minimum value of the measurement results.

Thereafter, the rail 1 was conveyed to a cooling bed and cooled in thecooling bed until the temperature reached room temperature to 100° C.and, thereafter, straightening was performed by a roller straighteningmachine to produce a rail 1 being a final product. Thereafter, the rail1 produced was cold sawn to thereby take a sample, and the hardness ofthe sample taken was measured. The sample was here taken at a pitch of 1m relative to the total length of the rail 1, and the Vickers hardnesstest was performed as hardness measurement at depth positions of 1 mmand 5 mm from the surface at the center in the width direction of thehead section 11 of the rail 1.

The cooling conditions and the evaluation results of material propertiesin the Conventional Examples are represented in Table 2. In ConventionalExamples 1 to 3 where the distance between the cooling headers and therail was 50 mm, the variation in temperature in the entire length waswithin 20° C. and the variation in hardness at each position where thesample was taken was also within an HV of 20 at a depth of 1 mm andwithin an HV of 10 at a depth of 5 mm. In Conventional Examples 4 to 9where the distance between the headers and the rail was 20 mm, thevariation in temperature in the entire length was within 120° C., thevariation in hardness was within an HV of 120 at a depth of 1 mm andwithin an HV of 60 at a depth of 5 mm, and material properties wereconfirmed not to be uniform. The reason for this was considered because,when the distance between the cooling headers and the rail was 50 mm,the influence of radiation from the rail 1 was smaller and therefore theamount of warpage of the cooling headers was smaller, and the variationsin temperature and hardness were smaller. On the other hand, it wasconsidered with respect to the condition where the distance between thecooling headers and the rail was 20 mm that the cooling headers wereheated by radiation of the rail 1 and thus the cooling headers werethermally deformed considerably and, therefore, the variations intemperature and hardness were larger.

When the distance between the cooling headers and the rail was 50 mm,however, a high pressure of 130 kPa exceeding 1 atm was required whenthe cooling rate is 7° C./sec to obtain a high-hardness structure.Therefore, such is not preferable in terms of facility cost and energycost. We confirmed from the foregoing that material properties uniformin the longitudinal direction were difficult to obtain, while a highcooling rate was obtained, in the conditions of Conventional Examples 1to 9.

TABLE 2 Distance between Target cooling Colling header length Conveyancedistance cooling headers rate at 5 mm L_(h) L_(o) and rail Spraypressure depth position Condition Component [m] [m] Coolant [mm] [kPa][° C./sec] Conventional Example 1 A 4 0 Air 50 130 7 ConventionalExample 2 A 4 0 Air 50 30 5 Conventional Example 3 A 4 0 Air 50 5 3Conventional Example 4 A 4 0 Air 20 30 7 Conventional Example 5 A 4 0Air 20 7 5 Conventional Example 6 A 4 0 Air 20 1.3 3 ConventionalExample 7 A 4 1 Air 20 30 7 Conventional Example 8 A 4 1 Air 20 7 5Conventional Example 9 A 4 1 Air 20 1.3 3 Variation in temperature aftercompletion of heat treatment Hardness at 1 mm depth position Hardness at5 mm depth position (Maximum-Minimum) Average Maximum Minimum AverageMaximum Minimum Condition [° C.] [HV] [HV] [HV] [HV] [HV] [HV]Conventional Example 1 20 393 400 387 375 380 370 Conventional Example 215 375 380 370 356 360 353 Conventional Example 3 10 357 360 353 338 340335 Conventional Example 4 120 360 400 320 350 380 320 ConventionalExample 5 110 343 380 307 333 360 305 Conventional Example 6 100 327 360293 315 340 290 Conventional Example 7 80 373 400 347 360 380 340Conventional Example 8 60 360 380 340 345 360 330 Conventional Example 940 347 360 333 330 340 320

Next, a rail 1 was produced in a condition where the conveyance distanceL_(o) of the example was adopted, as Example 1, and the materialproperties thereof were evaluated.

In Example 1, first, a bloom of each of chemical component compositionswith respect to A to C represented in Table 1 was cast by using acontinuous casting method. Herein, the balance of the chemical componentcomposition of the bloom was substantially Fe, and specifically Fe andinevitable impurities.

Next, the bloom cast was reheated to 1100° C. or more in a heatingfurnace, and thereafter taken out from the heating furnace and hotrolled through a break-down roller, a rough roller and a finish rollerso that the cross-sectional shape was the final shape, in the samemanner as in the Conventional Examples. In the hot rolling, the rail 1was rolled at an inverted position where the head section 11 and thefoot section 13 were in contact with a conveyance stage.

Furthermore, the rail 1 hot rolled was conveyed to the cooling apparatus2, and the rail 1 was cooled in the same manner as in the example (heattreatment step). Since the rail 1 was here rolled at the invertedposition as a rolling position, the rail 1, when carried in the coolingapparatus 2, was inverted, and allowed to be at an erect positionillustrated in FIG. 2, where the foot section 13 was located below inthe vertical direction and the head section 11 was located above in thevertical direction, and the rail 1 was restrained by clamps 23 a and 23b. Cooling was then performed by spraying of a coolant from each coolingheader. During such cooling, the coolant was any of air, mist or spraywater, and the distance between the cooling headers and the rail was 20mm. When the coolant was air, the spray pressure of the coolant was 5kPa to 50 kPa, and when the coolant was mist or spray water, 15% of aspray outlet was changed to a mist nozzle or a spray nozzle, and thecoolant was sprayed through such a nozzle at a spray pressure of 500 kPaor 300 kPa. When the coolant was mist or spray water, air was sprayedthrough 85% of the remaining outlet, and the pressure of air was 30 kPa.Cooling was performed with the spray pressure of the coolant beingchanged depending on the condition in the heat treatment step.Furthermore, cooling was performed in the heat treatment step until thesurface temperature of the head section 11 reached 530° C. or less,while temperature measurement was performed by the thermometer 24 in theapparatus, in the same manner as in the Conventional Examples.

Furthermore, cooling was performed in the heat treatment step inconditions of the length L_(h) of the cooling headers, where theconveyance distance L_(o) and the total conveyance distance (m) servingas the total distance of conveyance in cooling were changed within thescope of the example.

After completion of the heat treatment, the rail 1 was taken out fromthe cooling apparatus 2 onto the discharge table 4, and the surfacetemperature of the head section 11 of the rail 1 after cooling wasmeasured by use of the exit side thermometer 5 provided on the dischargetable 4, as illustrated in FIG. 5 and FIG. 6. The exit side thermometer5 was here used to measure the temperature at a plurality of positionsover the entire length in the longitudinal direction of the rail 1, andthe variation in temperature after cooling was calculated from themaximum value and the minimum value of the measurement results.

Thereafter, the rail 1 was conveyed to a cooling bed and cooled in thecooling bed until the temperature reached room temperature to 100° C.,and thereafter straightening was performed by a roller straighteningmachine to produce a rail 1 being a final product. Thereafter, the rail1 produced was cold sawn to thereby take a sample, and the hardness ofthe sample taken was measured. Herein, the sample was taken at a pitchof 1 m relative to the total length of the rail 1, and the Vickershardness test was performed as hardness measurement at depth positionsof 1 mm and 5 mm from the surface at the center in the width directionof the head section 11 of the rail 1.

The same manner was also conducted in Comparative Example 1 where thecondition of the conveyance distance L_(o) was different from that ofthe example, for comparison with Example 1, and material properties of arail 1 produced were evaluated.

The cooling conditions and the evaluation results of material propertiesin Example 1 and Comparative Example 1 are represented in Table 3. InTable 3, the pressure as the spray pressure condition of the coolant inExample 1-14 was changed from 10 to 30 at a position of ⅓ of the totalconveyance distance, and the pressure as the spray pressure condition ofthe coolant in Example 1-15 was changed from 30 to 10 at a position of ⅓of the total conveyance distance and the spray pressure was changed from10 to 30 at a position of ⅔ of the total conveyance distance. While theconveyance distance L_(o) was set to 4 m in the condition of ComparativeExample 1-3, conveyance was made by only up to 3.0 m during cooling ofthe rail 1, and while the conveyance distance L_(o) was set to 2 m inthe condition of Comparative Example 1-4, conveyance was made by only upto 1.0 m during cooling of the rail 1.

The variation in temperature in the entire length was within 20° C. inthe conditions of Examples 1-1 to 1-17, and the variation in temperaturein the entire length was smaller and was within 5° C. in the conditionwhere the oscillation distance L_(o) was n times the cooling headerlength L_(h). The variation in temperature, however, was within 20° C.or more in the condition where the oscillation distance L_(o) indicatedin Comparative Examples 1-1 to 1-4 was shorter than the cooling headerlength L_(h) or in the condition where the total conveyance distance inthe heat treatment was less than the cooling header length L_(h).

TABLE 3 Cooling header length Conveyance distance Total conveyance L_(h)L_(o) distance Spray pressure Condition Component [m] [m] [m] Coolant[kPa] Example 1-1 A 0.5 0.5 4.0 Air 30 Example 1-2 A 1 1 4.0 Air 30Example 1-3 A 2 2 4.0 Air 30 Example 1-4 A 4 4 4.0 Air 30 Example 1-5 A2 4 4.0 Air 30 Example 1-6 A 2 8 8.0 Air 30 Example 1-7 A 2 2 2.0 Air 30Example 1-8 A 2 2 5.0 Air 30 Example 1-9 A 2 2 2.5 Air 30 Example 1-10 B1 1 3.0 Air 30 Example 1-11 C 1 1 5.0 Air 30 Example 1-12 A 2 2 6.0 Air5 Example 1-13 A 2 2 8.0 Air 50 Example 1-14 A 2 2 10.0 Air 10→30Example 1-15 A 2 2 12.0 Air 30→10→30 Example 1-16 A 4 4 8.0 Mist 500Example 1-17 A 4 4 8.0 Spray water 300 Comparative Example 1-1 A 4 1 8.0Air 30 Comparative Example 1-2 A 2 1 8.0 Air 30 Comparative Example 1-3A 4 4 3.0 Air 30 Comparative Example 1-4 A 2 2 1.0 Air 30 Variation intemperature after completion of heat treatment Hardness at 1 mm depthposition Hardness at 5 mm depth position (Maximum-Minimum) AverageMaximum Minimum Average Maximum Minimum Condition [° C.] [HV] [HV] [HV][HV] [HV] [HV] Example 1-1 3 405 406 404 385 386 385 Example 1-2 3 400401 399 380 381 380 Example 1-3 4 390 391 388 370 371 369 Example 1-4 5360 362 359 350 351 349 Example 1-5 4 390 391 388 370 371 369 Example1-6 4 390 391 388 370 371 369 Example 1-7 4 390 391 388 370 371 369Example 1-8 17 385 391 380 367 371 363 Example 1-9 9 388 391 385 369 371367 Example 1-10 19 434 440 427 415 420 411 Example 1-11 19 444 450 437425 430 421 Example 1-12 4 370 371 368 350 351 349 Example 1-13 4 410411 408 390 391 389 Example 1-14 4 375 376 373 370 371 369 Example 1-154 395 396 393 390 391 389 Example 1-16 4 450 451 448 430 431 429 Example1-17 4 450 451 448 430 431 429 Comparative Example 1-1 80 344 371 318331 351 311 Comparative Example 1-2 40 378 391 364 361 371 351Comparative Example 1-3 35 359 371 348 342 351 334 Comparative Example1-4 25 383 391 374 365 371 359

We confirmed from the evaluation results of material properties that thevariation in temperature was suppressed within 20° C. or less, and thevariation in hardness was an HV of 13 or less at a depth position of 1mm from the surface and an HV of 10 or less at a depth position of 5 mmtherefrom in the conditions of Examples 1-1 to 1-17. On the other hand,the variation in temperature was not suppressed within 20° C. or less,and the variation in hardness was as large as an HV of 15 or more at adepth position of 1 mm from the surface and as large as an HV of 13 ormore at a depth position of 5 mm therefrom in the conditions ofComparative Examples 1-1 to 1-4.

In a comparison of the conditions indicated in Examples 1-1 to 1-9 whereComponent A was adopted, the spray pressure was constant and 30 kPa andthe coolant was air, we confirmed that the average hardness was as veryhigh as an HV of 391 or more at a depth position of 1 mm and was as veryhigh as an HV of 367 or more at a depth position of 5 mm in thecondition where the cooling header length L_(h) was 3 m or less. Theaverage hardness, however, was as low as an HV of 398 at a depthposition of 1 mm and as low as an HV of 379 at a depth position of 5 mm,while the variation in hardness could be reduced, in the condition wherethe cooling header length L_(h) was 4 m, as compared to the conditionwhere the cooling header length L_(h) was shorter.

We also confirmed in Examples 1-10 and 1-11 where the component waschanged, in Examples 1-12 and 1-13 where the spray pressure was changed,and in Examples 1-14 and 1-15 where the spray pressure was changedhalfway that the variations in temperature and hardness were reduced asin Examples 1-1 to 1-9. The average cooling rate in cooling was 4°C./sec in Example 1-12 where the spray pressure was the lowest, and theaverage cooling rate in cooling was 8.5° C./sec in Example 1-13 wherethe spray pressure was the highest. Therefore, we confirmed that, whenthe coolant is air, the desired effects can be exerted at least from 4°C./sec to 8.5° C./sec. We also confirmed that the variations intemperature and hardness were smaller, furthermore the average hardnessat a depth position of 1 mm was an HV of 479 and the average hardness ata depth position of 5 mm was an HV of 459, and the hardness was thusvery high regardless of a long cooling header length L_(h) of 4 m, inExamples 1-16 and 1-17 where the coolant was spray water or mist.

Example 2

Next, Example 2 is described. In Example 2, a bloom of a differentchemical component composition from that in Example 1 was used toproduce a rail 1 in the same manner as in Example 1 in the conditionwhere the conveyance distance L_(o) in the example was adopted, andmaterial properties of the rail 1 were evaluated. In Example 2, first, abloom of each chemical component composition of Conditions D to Frepresented in Table 4 was cast by using a continuous casting method.The balance of the chemical component composition of the bloom was heresubstantially Fe, specifically Fe and inevitable impurities.

TABLE 4 Chemical component composition (% by mass) Condition C Si Mn P SCr Sb Al Ti Others D 0.84 0.54 0.55 0.018 0.004 0.784 — — 0.002 V: 0.058E 0.82 0.23 1.26 0.018 0.005 0.155 0.0360 0.0001 0.001 F 0.83 0.66 0.260.015 0.005 0.896 0.1200 0.0005 0.001 Cu: 0.11, Ni: 0.12, Mo: 0.11 G0.82 0.55 1.13 0.012 0.002 0.224 — — — Nb: 0.009

Next, the bloom cast was reheated to 1100° C. or more in a heatingfurnace, thereafter hot rolled, and subsequently cooled (heat treatmentstep) in the same manner as in Example 1 described above. Measurement ofthe surface temperature of the rail 1 and cooling in the cooling bedafter completion of the heat treatment, and furthermore straighteningwith a roller straightening machine, sampling and hardness measurementwere also in the same conditions as in Example 1. The same manner wasalso conducted in Comparative Example 2 where the condition of theconveyance distance L_(o) was different from that of the example, forcomparison with Example 2, and material properties of a rail 1 producedwere evaluated.

The cooling conditions and the evaluation results of material propertiesin Example 2 and Comparative Example 2 are represented in Table 5.

TABLE 5 Cooling header length Conveyance distance Total conveyance L_(h)L_(o) distance Spray pressure Condition Component [m] [m] [m] Coolant[kPa] Example 2-1 D 2 2 4.0 Air 30 Example 2-2 E 1 1 4.0 Air 30 Example2-3 F 4 4 4.0 Air 30 Example 2-4 G 2 8 8.0 Air 30 Comparative Example2-1 D 4 1 8.0 Air 30 Comparative Example 2-2 G 2 1 8.0 Air 30 Variationin temperature after completion of heat treatment Hardness at 1 mm depthposition Hardness at 5 mm depth position (Maximum-Minimum) AverageMaximum Minimum Average Maximum Minimum Condition [° C.] [HV] [HV] [HV][HV] [HV] [HV] Example 2-1 4 479 480 478 409 410 409 Example 2-2 3 406407 405 474 475 474 Example 2-3 5 415 416 414 378 379 378 Example 2-4 4430 431 429 383 384 382 Comparative Example 2-1 81 481 508 455 410 430390 Comparative Example 2-2 39 432 455 409 382 392 372

The conveyance distance L_(o) was n times the cooling header lengthL_(h) in the conditions of Examples 2-1 to 2-4, and therefore thevariation in temperature in the entire length was within 5° C. and wassmaller. As a result, we confirmed that the variation in hardness was anHV of 2 or less at a depth position of 1 mm from the surface and an HVof 2 at a depth position of 5 mm therefrom in the conditions of Examples2-1 to 2-4.

On the other hand, we confirmed that the variation in temperature wasnot suppressed within 20° C. or less, and the variation in hardness wasas large as an HV of 40 or more at a depth position of 1 mm from thesurface and as large as an HV of 20 or more at a depth position of 5 mmtherefrom, in the conditions indicated in Comparative Example 2-1 to 2-2where the oscillation distance L_(o) was shorter than the cooling headerlength L_(h).

1-5. (canceled)
 6. A method of producing a steel material, wherein whena cooling apparatus having a plurality of cooling sections disposed sideby side in a longitudinal direction of a steel material cools the steelmaterial hot worked or cooled/reheated, the steel material is conveyedat a conveyance distance L_(o) (m) satisfying Equation (1), in onedirection along with the longitudinal direction of the steel material,in the cooling apparatus, wherein L_(o) is defined as conveyancedistance (m) of steel material, m is a natural number, and L_(h) isdefined as length (m) of cooling sections in longitudinal direction ofsteel material:(m−0.20)×L _(h) ≤L _(o)≤(m+0.20)×L _(h)  (1).
 7. The method according toclaim 6, wherein, in conveyance of the steel material, the steelmaterial is conveyed while being oscillated in both directions of onedirection and other direction along with the longitudinal direction ofthe steel material, and the amplitude of the oscillation is set to theconveyance distance L_(o) satisfying Equation (1).
 8. The methodaccording to claim 6, wherein the steel material is a rail material. 9.An apparatus that cools hot worked or cooled/reheated steel materialcomprising: a plurality of cooling sections disposed side by side in alongitudinal direction of the steel material; and a conveyance sectionconfigured to convey the steel material at a conveyance distance L_(o)(m) satisfying Equation (1), in one direction along the longitudinaldirection of the steel material in the cooling apparatus, during coolingof the steel material in the cooling sections, wherein L_(o) is definedas conveyance distance (m) of steel material, m is a natural number, andL_(h) is defined as length (m) of cooling sections in longitudinaldirection of steel material:(m−0.20)×L _(h) ≤L _(o)≤(m+0.20)×L _(h)  (1)
 10. A steel materialproduced by hot working or cooling/reheating and thereafter cooling in acooling apparatus having a plurality of cooling sections disposed sideby side in a longitudinal direction, wherein, during cooling in thecooling apparatus, the steel material is produced while being conveyedat a conveyance distance L_(o) (m) satisfying Equation (1), in onedirection along with the longitudinal direction of the steel material inthe cooling apparatus, wherein L_(o) is defined as conveyance distance(m) of steel material, m is a natural number, and L_(h) is defined aslength (m) of cooling sections in longitudinal direction of steelmaterial:(m−0.20)×L _(h) ≤L _(o)≤(m+0.20)×L _(h)  (1).
 11. The method forproducing a steel material according to claim 7, wherein the steelmaterial is a rail material.